Add get_attributes_mut for Mesh

[hukasu]

Objective

Closes #2134

Solution

Create a method in Mesh to retrieve multiple attributes mutably.

Testing

New test on bevy_mesh/mesh.rs

Alternatives

Leave to the users collect the desired attributes through the Mesh::attributes_mut method

Showcase

let [pos1, pos2, pos3, normal, color] = mesh.get_attributes_mut([
    &Mesh::ATTRIBUTE_POSITION.id,
    &Mesh::ATTRIBUTE_POSITION.id,
    &Mesh::ATTRIBUTE_POSITION.id,
    &Mesh::ATTRIBUTE_NORMAL.id,
    &Mesh::ATTRIBUTE_COLOR.id,
]);
assert_eq!(pos1.unwrap().0, &Mesh::ATTRIBUTE_POSITION);
assert!(pos2.is_none());
assert!(pos3.is_none());
assert_eq!(normal.unwrap().0, &Mesh::ATTRIBUTE_NORMAL);
assert_eq!(color.unwrap().0, &Mesh::ATTRIBUTE_COLOR);

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[IceSentry]

I thinks this could be useful, can you fix the conflicts?

[IceSentry]

Is it possible to avoid this unimplemented branch? I'd prefer if it returned and error instead of panicking.

[hukasu]

that is actually supposed to be an unreachable